1. Field of the Invention
The invention relates generally to PIN photodiodes. More particularly, the invention relates to a PIN photodiode semiconductor structure diode which has high reliability, with extremely low dark current, and suitable to be used in high temperature, high humidity environments, and processes for fabrication of such devices.
2. Description of the Related Art
Fiber optic communications typically employ a modulated light source, such as a laser, a photodiode light detector, and an optical fiber interconnecting the laser and the photodiode. The laser is modulated to emit light pulses that are transmitted over an optical fiber and received at a remote unit that includes a photodiode to convert the optical signal into an electrical signal. In particular, PIN diodes are widely used the photodiodes in the optical receiver for high-speed fiber optics communication. A typical lateral PIN diode fabricated by Zn diffusion has an exposed PN junction on the top surface. Traditionally, the exposed PN junctions on the surface of the devices are covered by dielectric for passivation. However, the dielectric protection often degrades under various reliability test environments such as high temperature, high humidity test (HTHH), etc. One of the most commonly used methods is Zn diffusion, where Zn atoms are diffused through a window layer and forms an active PN junction just inside the absorbing layer. A typical Zn diffused PIN photodiode will also create a side PN junction on the surface of the window layer, which is also a semiconductor material.
Patterned area Zinc (Zn) diffusion has been widely employed for fabricating InP based high-speed PIN photodiodes. The commonly used epitaxial structure of the photodiode before diffusion is the layer sequence n-InP, intrinsic InGaAs and n++InP 103 (or with an undoped InGaAs top layer). In the traditional process, a dielectric layer is first deposited on top of the InP or the InGaAs layer and diffusion opening formed. The p-type doping is achieved by diffusing the Zn atoms directly (or via InGaAs layer) into the InP layer 103 through the diffusion openings in the dielectric layer. The final structure after diffusion is p-InP 104; Intrinsic InGa 102 and n++InP 101 PIN, which is typical for a PIN photodiode.
One of the drawbacks for the traditional process is that there is an exposed PN junction on the surface of the top InGaAs or InP layer. Without re-depositing a dielectric layer for high quality passivation, the surface recombination current is potentially high, which gives rise to inconsistent operating dark current after fabrication. Even with good initial leakage characteristics, significant degradation of operation dark current often exists after aging, especially under conditions of high temperatures operating live (HTOL) tests of high temperature/high humidity reverse biased tests (THRB), where dielectric protection is normally weak if attacked by corrosion.
Prior to the present invention, there has not been suitable means for protecting the PN junction in a PIN diode fabricated by Zn diffusion without the use of an additional dielectric sealing layer in the PIN photodiode.